By structural diaphragm we mean, in short, a trench of great depth configured to isolate a certain portion of soil. The trench, which can have a variable thickness of between a few tens of centimetres and a few meters, can be even hundreds of meters long. Such a trench is made by digging a plurality of rectangular sectors in sequence. Each of these rectangular sectors is filled with cement mixture and, if necessary, can be reinforced with a steel cage or with IPE beams.
The equipment mainly configured for digging the rectangular sectors that form a structural diaphragm are hydraulic or cable-operated buckets and milling cutters. Buckets and milling cutters both have the feature of being hung from a carrying machine through a cable unwound from a winch. Such a carrying machine generally consists of a tracked undercarriage, a turret rotating with respect to the carriage and an arm able to tilt with respect to the turret, on which the bucket or the milling cutter is hung. Conventionally, the machine is a crane or a driller. The body of the bucket and/or of the milling cutter is sufficiently long and heavy to self-guide into the soil being dug, as if it were a pendulum. In some cases, in the presence of certain geological configurations or deep excavations, such buckets and milling cutters can be provided with means for measuring the deviation and with verticality correction devices, commonly known in the field as flaps, grip rollers, shoes, etc.
In particular, a bucket is provided, in the lower part of its body, with a pair of half-shells or jaws that provide the rectangular digging section. These half-shells are driven by a system of cables and pulleys in the so-called cable-operated or mechanical bucket, and by a hydraulic piston in the hydraulic bucket. The extraction of the debris is carried out by lifting the entire bucket from the bottom of the excavation up to ground surface level, where such a bucket is emptied, usually directly onto a dumper.
Milling cutters are more mechanically complex and more expensive with respect to buckets because they are equipped with cutting wheels and hydraulic pumps for sucking up debris and their use requires more hydraulic power. Milling cutters, since they are heavier than buckets, offer better guarantees of verticality but their use is only advantageous in hard ground, in which they perform better than buckets, and in very deep excavations.
Buckets, on the other hand, are simpler and more cost-effective than milling cutters in terms of their production and subsequent maintenance. Buckets require less power than milling cutters, but they have the drawback of reaming the walls of the hole made during every transit step both going down and coming up (the excavation is of the discontinuous type). They have a relatively limited storage capacity during each individual operating cycle. In hard ground, moreover, the forward movement of a bucket is extremely limited and must be aided with the help of bits and grapnels. Finally, it is clear that a bucket becomes less effective as the depth of the excavation increases, since it also increases the time taken to obtain an ever increasing volume of material extracted.
Irrespective of whether or not it is advantageous to use a bucket rather than a milling cutter, it should be noted that current buckets are not free of drawbacks. The bucket, since it has to be inserted and extracted many times into the excavation in order to reach the desired depth, must necessarily be simple in use and in construction. During ascent and descent, in addition to winding up and unwinding the support cable, it is also necessary to wind up and unwind all of the hydraulic tubes and electrical cables that drive the actuators of the bucket and this involves mechanical complications, greater wear, greater exposure to damage and additional costs. In most cases this means that it is preferred to supply just the cylinder that drives the half-shells and that, in some cases, cable-operated mechanical buckets are preferred. The depth of 40-70 meters is conventionally the one which defines this virtual limit of advantageousness, considering that when excavations become deep there is a need to equip buckets both with additional equipment to control verticality, and with correction flaps driven by hydraulic actuators. The aforementioned considerations are also based on the analysis of how depth influences the times of the operating cycle of a standard bucket, which is carried out in six distinct steps:    1) positioning on the excavation;    2) descent into the stabilizing fluid (if present) down to the bottom of the excavation;    3) partial ascent, release into free fall of the cable so that the half-shells penetrate into the bottom of the excavation, closing of the half-shells and collection of the soil to be removed (active step); this step can be repeated many times depending on the type of soil and the ease of filling;    4) ascent from the bottom of the excavation with the half-shells full with soil until the bucket has been completely extracted from the excavation;    5) rotation of the carrying machine in the direction of the dumper or of the pile;    6) unloading of the bucket.
In order to reach the desired digging depth, the aforementioned cycle must be repeated a number of times that is proportional to the volume of soil that can be removed in each cycle. Steps 1, 3, 5 and 6 last the same time irrespective of the depth reached in the excavation. Steps 2 and 4, on the other hand, have a duration that is proportional to the depth of the excavation. In the first meters the depth of the excavation has practically no influence on the cost-effectiveness of the single operating cycle, but as the depth increases the duration of steps 2 and 4 tends to exceed, even greatly, the sum of the duration of the other four steps.
There are margins of improvement, even if they are rather small. The ascent step is regulated by the speed of the winch, but the closed bucket loaded with debris that rises along the excavation full of stabilizing liquid behaves like the piston of a syringe. Therefore, it is not suitable to excessively increase the speed of ascent of the bucket, since it would promote a sucking effect that could compromise the stability of the walls of the excavation.
The descent step leaves some margin of intervention. By creating suitable openings and discharges in the structure of the bucket or half-shells, i.e. by attending to the hydrodynamics of the planes and surfaces, it is possible to facilitate the outflow of stabilization fluid through the bucket itself, so as to reduce the descent time into the excavation, but the gain would not be very appreciable (see document EP 2 484 837 A1, described in greater detail hereafter).
It may be more suitable to optimise the load capacity of the bucket, attempting to increase the amount of material extracted during each single operating cycle. In this way, each cycle would become more economically profitable, at the same time reducing the number of cycles to make an excavation of predetermined depth, by virtue of the increased storage volume.
In the state of the art attempts have been made to reduce the unproductive times of the operating steps of buckets, as well as to increase the storage capacity that can be exploited in every single cycle. For example, document EP 2 484 837 A1 proposes to improve the hydrodynamics of an empty bucket in its descent towards the bottom of the excavation, thanks to the presence of openings or holes obtained in the top of the open half-shells. This characteristic should facilitate the outflow of stabilization fluid of the excavation from below to above the bucket. The size of these openings or holes is however limited by the geometry of the half-shells and therefore the reduction in friction is minimal, just as the reduction in descent time of the bucket is minimal.
Document EP 1 614 813 A1 in the name of the same Applicant proposes a bucket-equipped apparatus still hung from a cable and configured to be dropped into an excavation, but in which the bucket is made up of four tubes of large diameter, welded tangentially to each other so as to be configured in a rectangle that represents the dimensions of the excavation to be made. The tubes are arranged in the excavation in the vertical direction. Every tube, of a length of a few meters, carries a hydraulic motor at its top, which sets a helix element that is as long as the tube and that projects beneath the tube itself into right-handed rotation. Each helix is equipped with teeth in its lower part. The helixes, in the portion outside the tube, are interpenetrating so as to make an excavation comparable to four slightly intersecting circumferences. The helixes, in their rotation motion, carry the dug material inside the tubes. When the apparatus is full, it is extracted from the excavation and it is emptied, rotating the helixes in the anti-clockwise direction.
This kind of apparatus it thus intended to make excavations of equivalent section to that of a standard bucket, but exploiting the volume represented by the height of the tubes, able to hold more than the half-shells, in order to be able to carry more material in each cycle. In reality, such an effect is obtained only in reduced form, particularly in the presence of loose sands, due to the presence of the stabilization fluid of the hole. Indeed, in practice, the volume of the extracted material is only a fraction of the theoretical volume since the flow of stabilizing liquid, which passes through the framework of the apparatus, which is not really hydrodynamic, disperses a great deal of the dug material, which falls to the bottom of the excavation. Such an apparatus also has the drawback of taking longer to be filled, particularly in the presence of cohesive soil. Moreover, it is necessary to make the hydraulic plant more complicated and to have high power to supply the motors of the helixes.